Packaging materials used for foods and medical goods should have such properties (gas barrier properties) of interrupting the permeation of water vapor, oxygen and other gases capable of causing content quality to be deteriorated so as to suppress the content from being deteriorated or decayed and retain the function and properties of foods and medical goods.
Therefore, these packaging materials have been hitherto provided with a gas barrier layer made of a material having gas barrier properties. Until now, the gas barrier layer has been formed on a base material such as a film or paper according to a sputtering or vacuum deposition method, or a wet coating or printing method. The gas barrier layer used thus far includes a metal foil or metal deposition film made of a metal such as aluminum, or a resin film such as of polyvinyl alcohol, an ethylene/vinyl alcohol copolymer, polyvinylidene chloride or the like (see, for example, Patent Literatures 1 to 5).
However, although the metal foil or metal deposition film is excellent in gas barrier properties, they have many problems in that a content cannot be confirmed because of opaqueness thereof, cracks occur by several percent elongation owing to poor elasticity thereby causing gas barrier properties to be lowered, and they have to be treated as an incombustible waste at the time of disposal after use.
The gas barrier layer made of a resin film such as of polyvinylidene chloride shows humidity dependence-free, good gas barrier properties, but with the possibility that it becomes a generation source of harmful substances such as dioxins when subjected to waste disposal treatment. Thus, packaging materials including chlorine-based materials tend to be hated.
On the other hand, although a gas barrier layer formed of a resin film such as of polyvinyl alcohol or ethylene/vinyl alcohol copolymer shows high gas barrier properties in an atmosphere of low humidity, the layer is humidity-dependent in nature, with the attendant drawback that the gas barrier properties lower greatly as temperature rises.
With other types of gas barrier resin films, the gas barrier properties are inferior to those of a polyvinylidene chloride resin film, or a polyvinyl alcohol resin film in an atmosphere of low humidity.
In order to improve the gas barrier properties of these resin films, there have been proposed composite resin films of resins and inorganic layered minerals. With the composite resin film, it is necessary that an inorganic layered mineral be distributed and arranged in order in the inside of the film so as to allow improved gas barrier properties. However, as the inorganic layered mineral is distributed and arranged in order, the cohesive force of the resin film and the adhesion force of the film to a base material lower. Thus, it is very difficult to satisfy both of high gas barrier properties and adhesion strength sufficient for use as a packaging material.
Besides, a proposal has been made wherein an aqueous polyurethane is added to the above-mentioned composite material of polyvinyl alcohol or ethylene/vinyl alcohol copolymer and a layered compound thereby providing a gas barrier film having an improved adhesion of the composite material to a base material (see, for example, Patent Literature 6). However, this gas barrier film has been such that the gas barrier properties at a high humidity of not less than 80% are not satisfactory.
On the other hand, there has been proposed a gas barrier resin laminate film that is comprised of a polyurethane resin having a high concentration of an urethane group or urea group and a polyamine and is good in adhesion to a base material and low in humidity dependence (see, for example, Patent Literature 7). However, this gas barrier film is inferior to a resin film made of the above-indicated polyvinylidene chloride with respect to gas barrier properties and thus, limitation is placed on its application as a gas barrier packaging material.
With a laminate film wherein an adhesive is coated onto a film side (or on a film) on which another type of film is bonded, there is required laminate strength sufficient not to cause degradation over time.